This invention relates to commutators for dc machines.
A conventional dc machine comprises an armature and a stator. The armature includes a shaft, a core of slotted steel laminations which are rotated by the shaft, and copper windings which are inserted in the slots. The stator includes a number of field windings which surround the armature. When the field windings are energized, the resulting stator flux cuts through the laminations of the rotor. Because the stator is stationary, its mmf vector is fixed in space.
When the dc machine is operated as a motor, the armature windings are selectively energized such that the resulting armature mmf vector is maintained at a torque angle of 90.degree. (quadrature) with the stator's mmf vector. Because the stator mmf vector is fixed in space, the armature mmf vector must also be held stationary. To keep the armature mmf vector stationary while the armature rotates, the polarity of flux lines from the armature must be continuously reversed.
Polarity of the flux lines is reversed by changing direction of the current in the armature windings. The direction is changed by a commutator, which consists of conductive bars that are insulated from each other. The start of each armature winding is connected to one bar, and the finish is connected to another bar. Brushes, urged against the surface of the commutator, carry current to the commutator bars. As the armature rotates, different bars contact the brushes, whereby the direction of current in the armature windings is reversed to keep the armature mmf vector in quadrature with the stator mmf vector.
With time and usage, the brushes wear down and the commutator surfaces run out. Therefore, the brushes must be replaced; otherwise, the motor runs noisier and eventually fails. Further, worn brushes emit electrical interference caused by arcing. The arcing also causes undesirable effects such as acoustic noise and sparking.
These problems have been overcome by recent advances in dc motor designs. In newer brushless dc motors, the armature is replaced by a permanent magnet rotor and the commutators and brushes are replaced by power transistors and electronic control circuitry. By modulating the power transistors, the direction of current can be commutated in the stator windings, causing the stator mmf vector to rotate. Thus, commutation is performed electronically instead of mechanically. As a result of these advances, the brushless dc motor has become dominant in servo motor applications such as robotics and machine tools.